When a GRE packet is received over any network IP interface, the router checks if destination address matches the system interface address (exact match) or the GRE termination subnet (Longest Prefix Match). The router then processes the packet according to the following criteria:
If a match exists and the GRE Protocol Type field indicates an MPLS payload, continue processing the MPLS label stack as normal. This includes:
Pop one or more labels and forward to CPM if a MPLS exception exists (TTL expiry, RA label, 127/8 destination address in underlying IP packet).
Pop one or more labels and look up the packet in the FIB or in a local service context. The router operates as an egress LER.
Pop one or more labels and swap a label out to the outgoing interface with NHLFE encapsulation pushed on the packet. The router operates as an LSR.
When the incoming label is swapped to an implicit-null label, the user is able to remark the DSCP field of the exposed IPv4 or IPv6 packet on egress of the data path.
If a match exists and the GRE Protocol Type field indicates an IPv4 or an IPv6 payload, continue processing in the pipeline as an IP packet and forward out based on FIB lookup.
If a match exists and the GRE Protocol Type field indicates a Bridged Ethernet payload, drop the packet. To enable the feature to terminate the Bridged Ethernet payload, ensure that the termination subnet for that feature does not overlap with the GRE termination subnet of MPLS-over-GRE and IP-over-GRE termination.
If a match exists and the GRE protocol Type field is set to any other payload value, drop the packet.
If a match exists and the packet is not dropped, the application of ACL filter on the incoming interface matches against the inner (payload) header of the received GRE-encapsulated packet.
If a match does not exist, continue processing in the pipeline as an IPv4 packet. In this case, the application of ACL filter on the incoming interface matches against the outer IPv4 header of the received GRE-encapsulated packet.
This feature supports GRE/IPv4 encapsulation when the payload is MPLS, IPv4, or IPv6.
All MPLS egress LER and LSR features associated with the processed label are supported.